A wireless local area network (WLAN) basically supports an access point (AP) in a distributed system (DS) and a basic service set (BSS) including a plurality of wireless stations, which are not an access point.
A Medium Access Control (MAC) protocol of a WLAN operates based on a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). Accordingly, the WLAN disadvantageously wastes resources in channel contention. In order to overcome such a problem, an enhanced MAC protocol defined in IEEE 802.11a introduced a burst transmission scheme. In the burst transmission scheme, multiple MPDUs are transmitted at an interval of Short Inter-Frame Space (SIFS) during a Transmission Opportunity (TXOP) time after obtaining a transmission right of a wireless resource and ACKs are received in response to the transmitted MPDUs.
Further, Aggregated MSDU (A-MSDU) and Aggregated MPDU (A-MPDU) are defined in IEEE 802.11 n. More than one MSDUs are tied together without an Inter Frame Space (IFS) and transmitted using one time wireless resource through contention.
Lately, WLAN users have been abruptly increased. IEEE 802.11 ac has been standardizing a Very High Throughput (VHT) WLAN system in order to increase data throughput provided by one BSS.
A VHT WLAN system supports, in a multiple (three)-STA environment consisting of one AP and two STAs, maximum throughput of 1 Gbps at the MAC SAP of the AP, as well as maximum throughput of 500 Mbps at the MAC SAP of a wireless STA for point-to-point environments. There is also consideration made so that each AP and STA of the VHT WLAN simultaneously supports compatibility with existing WLAN (IEEE 802.11a/n system).
Meanwhile, when a wireless STA acquires a TXOP in a wireless communication system (e.g. WLAN), the STA needs to receive a response through a response frame regarding a request frame in order to improve reliability of wireless communication. For example, frames requiring responses include a RTS (Request-To-Send) frame, a BAR (Block ACK Request) frame, a data frame, and frames for various requests (e.g. probe request, authentication request, association request). Response frames responding to the frames requiring responses are as follows: a response frame responding to a RTS frame is a CTS (Clear-To-Send) frame; a response frame responding to a BAR frame is a BA (Block ACK) frame; a response frame responding to a data frame is an ACK or BA frame; and response frames responding to various request frames are frames of various responses (e.g. probe response, authentication response, association response).
The responses are classified into immediate responses and delayed responses. Responses (ACK frames) to a single piece of data correspond to the immediate responses; and block responses responding to a continuous transmission or aggregated MPDU correspond to both immediate and delayed responses.
An immediate response is used in the following manner: when PHY-RXEND.primitive of a received request frame is generated, a response frame is transmitted after SIFS so that other wireless STAs do not transmit. In this case, the frame exchange sequence constitutes a pair, as described above. The generated response frame may not include a transmission address.
On the other hand, a delayed response is used in the following manner: a response is made through an ACK frame as a basic response for informing of whether an initially generated request frame has been received or not, and a response frame including requested information is then transmitted. The response frame in this case may be transmitted through EDCA (Enhanced Distributed Channel Access) regarding channels, piggybacked by another frame, or aggregated and transmitted together with another frame. According to the delayed response scheme, a transmitting STA receives a response frame from a receiving STA and then informs the receiving STA that the response frame has been received using an ACK frame.
The request and response frame exchange sequence in such a WLAN may be applied to a multiple user wireless communication system. When uplink MU-MIMO technology is supported, wireless STAs can receive frames according to the above-mentioned frame exchange sequence and simultaneously transmit response frames after IFS.
When uplink MU-MIMO technology is not supported, or when better throughput is desired even if uplink MU-MIMO technology is supported, a wireless STA, after receiving a frame, needs to transmit a response frame using a difference of channel or time.
Schemes for exchanging frames using a difference of time but the same bandwidth in a WLAN system include a scheme of exchanging a request frame eliciting response frames and a response frame through channel access for each wireless STA, and a scheme of transmitting, by a plurality of wireless STAs, subsequent response frames through a single request frame.
To be specific, the scheme of exchanging a request frame eliciting response frames and a response frame through channel access for each wireless STA is as follows: an AP transmits a request frame to a first wireless STA through channel access; the first wireless STA transmit a response frame after SIFS; the AP transmits another request frame to a second wireless STA through channel access; and the second wireless STA transmits a response frame after SIFS.
The scheme of transmitting, by a plurality of wireless STAs, subsequent response frames through a single request frame is as follows: a single request frame including address information regarding multiple users is transmitted through channel access, and the wireless STAs then subsequently transmit response frames after SIFS.
IEEE 802.11 standards recommend that, in order to avoid collision between frames on a wireless path, control frames are transmitted to be receivable by all STAs belonging to the corresponding BSS. In a MU-MIMO-based VHT WLAN system, control frames need to be transmitted to be receivable by all STAs, even if a plurality of communication paths are used. The control frames in this connection include RTS, CTS, ACK, BAR, BA, and various poll frames.
When a plurality of communication paths are used simultaneously, STAs receiving control frames may have one-to-one correspondence to respective communication paths, meaning that that a control frame must be transmitted through each communication path. However, this increases the rate of overhead concerning transmission/reception of control frames, degrading the advantage of using MU-MIMO technology.